The present invention relates to a control system for establishing and maintaining a desired uniform temperature within an enclosure, more particularly to such a control system which controls the supply of fluid to a heater in order to maintain a substantially uniform temperature.
Temperature control systems are, of course, well known and typically include a device for selecting a desired temperature, such as a thermostat, a device for sensing the actual temperature, and a mechanism for activating and deactivating a heating device. Such known systems will activate one or more heating devices when the actual temperature is below the desired temperature and will deactivate one or more heating devices when the actual temperature rises above the desired temperature. In order to prevent excessive on/off cycling of the heating devices, the known temperature control systems will typically establish on and off set points located below and above the desired temperature. While such known systems eliminate excessive on/off cycling of the heating device, they inherently result in a fluctuation of the actual ambient temperature between the on and off set points.
FIG. 1A illustrates the temperature variation over time for such known systems. As can be seen, the actual ambient temperature varies between the ON set point and the OFF set point and fluctuates above and below the desired temperature. The output of the heating device versus time for the known system is graphically illustrated in FIG. 1B. The output varies cyclically between 0% and 100% in response to input signals from the thermostat.
This temperature fluctuation not only results in discomfort, but in certain circumstances, is highly undesirable. Specifically, in the livestock and poultry industries, excessive variations in the temperature within the animal and fowl enclosures have been found to have adverse affects on the growth of the animals and fowl by affecting the food intake, and inducing animal and fowl stresses. Many experimental research facilities require the maintenance of a uniform temperature to minimize the effects of temperature variations on their ongoing research. Even in typical household or office environments, excessive temperature variations results in discomfort and detracts from the enjoyment of those surroundings.
Systems are known that include a variable valve to control a fluid being supplied to one or more heating devices with a capillary driven actuator having a manual temperature set point adjustment. Internal fluid in the capillary tube expands or contracts in accordance with the ambient temperature driving the actuator to open or close the variable valve until equilibrium is found that matches the manually set point. This system is limited insofar as the set point can only be manually adjusted and the placement of the temperature sensing capillary tube is limited to within a close vicinity of the capillary driven actuator.
Other systems are known which use an electric motor and a gear drive to move a piston which adjusts a pressure regulator to control the fluid to the one or more heating devices. Although this system can be remotely controlled, or manually adjusted, the remote control requires a variable voltage or current input. The variable inputs limit the number of devices to which it can be connected, and the remote control unit must account for the delays in the system from changing the pressure of the fluid supplied to the one or more heating devices to the resultant temperature change. This system also encompasses a number of moving parts which often limit the useful life of the system.
A standard ball valve may be driven by an electric stepper motor to vary the fluid flow to the heating device. However, a system including the valve and stepper motor actuator cannot maintain a steady state pressure within the fluid being supplied to the one or more heating devices when the heating devices are shut off, or when the flames in the one or more heating devices are extinguished.
It is also possible to utilize multiple solenoid valves to select between different pressure regulators in order to regulate the pressure of the fluid being supplied to the one or more heating devices. However, these systems are limited to a step change in the pressure of the fluid and do not provide a continuously variable change in the pressure. This system also requires a number of solenoids and pressure regulators which render the system inherently complex and less reliable than is desired.
The present invention relates to a control system for establishing and maintaining a desired uniform temperature in a room heated by one or more heating devices. The control system includes a valve assembly for controlling the pressure of the fluid supplied to the heating device, the valve assembly including a valve actuator, a pressure sensor sensing the pressure of the fluid supplied to the heater, and a temperature control device, such as a thermostat, or electronic temperature controller to sense the ambient temperature of the room and to compare the sensed ambient temperature to a predetermined desired temperature. The control system also includes an electronic control unit connected to the aforementioned elements, the electronic control unit including an electronic circuit which receives a signal from the temperature control device, determines the rate of change of the pressure of the fluid from a predetermined pressure set point, and varies the pressure set point (which thereby varies the heating device output), by providing a command signal to the valve actuator based upon an xe2x80x9copenxe2x80x9d or xe2x80x9cclosexe2x80x9d signal from the temperature control device. Thus, the command signal from the electronic control unit is indicative of the difference between the predetermined desired temperature and the actual temperature, and the rate of change of the pressure of the fluid. The temperature control device asserts an xe2x80x9copenxe2x80x9d signal (heat required) if the ambient temperature is below the desired temperature, and asserts a xe2x80x9cclosexe2x80x9d signal if the ambient temperature is above the desired temperature.
The control system according to the present invention limits the frequency at which high and low room temperatures are reached by providing a continuous regulated heat flow that is gradually increased, or gradually decreased as needed to maintain the desired temperature set point.
The valve in the fluid supply line which supplies fluid to the heating device is controlled by a thermal heat motor that is designed to be used as an on/off controller for the valve. The electronic control unit monitors the fluid supply line pressure at predetermined intervals to provide a closed loop algorithm for increasing or decreasing the pressure of the supplied fluid and regulating the power to the heat motor. The heat motor is regulated by a combination of pulse width modulation (PWM) and stepped on/off pulses. A nominal power is applied to the thermal heat motor which maintains the output control in a desired position. When the pressure of the supplied fluid should be increased (temperature control device asserting xe2x80x9copenxe2x80x9d signal) to increase the output of the heating device, the applied power to the heat motor is doubled for a predetermined amount of time, and the level of the nominal power to the heat motor is incremented by 1 part in 256. When the pressure of the supplied fluid should be decreased (temperature control device asserting xe2x80x9cclosexe2x80x9d signal) in order to decrease the output of the heating device, the applied power to the heat motor is reduced to zero for a predetermined amount of time and the nominal power to the heat motor is decremented by 1 part in 256. Alternatively, there are other methods to implement the electronic control unit. A digital/analog (DIA) connector could provide a control voltage to the heat motor. Also, the resolution need not be exactly 1 part in 256, but may be changed according to the parameters of each particular installation. The nominal power to the heat motor is controlled by the PWM duty cycle, which slowly and automatically compensates for the changes in ambient temperature which affect the heat motor.
The control system according to the present invention utilizes two feedback control loops. One loop controls the pressure of the fluid being supplied to the heating device, and the other loop controls the temperature by adjusting the pressure set point of the fluid pressure control loop. The pressure control loop monitors the pressure at predetermined intervals and, if the pressure of the fluid should be changed, it is changed by incrementing or decrementing the PWM register.
The temperature control loop controls the temperature by adjusting the pressure set point of the pressure control loop. The ambient temperature is monitored at predetermined intervals and, if the temperature is above the temperature set point, a xe2x80x9cclosexe2x80x9d signal is asserted by the temperature control device and the fluid pressure set point is decreased by a predetermined amount. If the ambient temperature is below the temperature set point, an xe2x80x9copenxe2x80x9d signal is asserted by the temperature control device and the fluid pressure set point is increased by a predetermined amount.